Bolometers are energy detectors based upon a change in the resistance of materials (called bolometer elements) that are exposed to a radiation flux. The bolometer elements have been made from metals or semiconductors. In metals, the resistance change is essentially due to variations in the carrier mobility, which typically decreases with temperature. Greater sensitivity can be obtained in high-resistivity semiconductor bolometer elements in which the free-carrier density is an exponential function of temperature, but thin film fabrication of semiconductor for bolometers is a difficult problem.
FIGS. 1 and 2 are a cross sectional and a perspective views illustrating a two-level bolometer 10, disclosed in U.S. Pat. No. 5,300,915 entitled "THERMAL SENSOR", the bolometer 10 including an elevated microbridge detector level 11 and a lower level 12. The lower level 12 has a flat surfaced semiconductor substrate 13, such as a single crystal silicon substrate. The surface 14 of the silicon substrate 13 has fabricated thereon several components of an integrated circuit 15 including diodes, x and y bus lines, connections, and contact pads at the ends of the x and y bus lines, the fabrication following conventional silicon IC technology. The integrated circuit 15 is coated with a protective layer of silicon nitride 16. The valley strip 17 is the area not covered by the elevated detector.
The elevated detector level 11 includes a silicon nitride layer 20, a serpentine metallic resistive path 21, a silicon nitride layer 22 over the layers 20 and 21, and an infrared absorber coating 23 (hereinafter, "IR absorber coating") over the silicon nitride layer 22. Downwardly extending silicon nitride layers 20' and 22' deposited at the same time during the fabrication make up the four sloping support legs for the elevated detector level 11. The number of support legs may be greater or less than four. The cavity 26 between the two levels is ambient atmosphere. During the fabrication process, however, the cavity 26 was originally filled with a previously deposited layer of easily dissolvable glass or other dissolvable material until the layers 20, 20' and 22, 22' were deposited. Subsequently in the process the glass was dissolved out to leave the cavity.
In FIG. 3, there is a top view depicting the elevated detector level 11 shown in FIG. 1. This drawing is made as though the overlying absorber coating 23 and the upper silicon nitride layer 22 are transparent so the serpentine resistive layer path 21 can be shown. The ends 21a, 21b of the resistive path 21 are continued down the slope area 30 to make electrical contact with pads 31 and 32 on the lower level 12. FIG. 3 shows the nitride window cuts 35, 36 and 37 which are opened through the silicon nitride layers 20 and 22 to provide access to the phosphor-glass beneath for dissolving it from beneath the detector plane. The nitride window cuts 35, 36, 37 to provide this access are narrow and are shared with adjacent pixels on the sides, thus maximizing the area available to the detector and thus maximizing the fill-factor. The four supporting bridges may be short or as long as necessary to provide adequate support and thermal isolation.
One of the shortcomings of the above described bolometer is its less than optimum fill factor resulting from the presence of the bridges on same level as the elevated microbridge detector level, which, in turn, reduces the total area for IR absorbing, i.e., the fill factor.